1. Field of the Invention (Technical Field)
The present invention relates to spacecraft propulsion, particularly to micropropulsion.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications may not be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Throughout most of the history of space propulsion, the emphasis has been on producing rocket engines with ever larger thrust. Now, with the advent of microsatellites ( greater than 10 kg), nanosatellites (1-10 kg) and even picosatellites ( less than 1 kg), this trend is reversing in one branch of motor development. The move toward micro- and nano-satellites in the 1 to 10-kg mass range places severe challenges on conventional technologies for orientation thrusters. It is obvious that these need to be efficient, extremely lightweight, long-life devices with high specific impulse (Isp), which is the exhaust velocity divided by the acceleration of gravity.
A motor on a 10-cm lever arm producing a minimum impulse bit (MIB) of 1 nano N-s will steer a 1 kg satellite with angular velocity bits of order 25 nrad/sec. This impulse is an order of magnitude less than the momentum developed by a flying mosquito.
Desirable features of such a steering engine are: Mass xe2x89xa6200 g; Size of order 1xc3x975xc3x9710 cm; High specific impulse; 0.1 millidyn-s MIB; 10,000:1 single impulse dynamic range; Continuous thrust 10 dyn (0.1 milli-N); Very large thrust dynamic range; Power consumption 3 W; Very low off-state power; Direct spacecraft bus voltage input; No nozzle; Lifetime impulse (1000 N-s) sufficient to re-enter a 10 kg satellite from Low Earth Orbit (LEO); Inexpensive, common, nonpolluting fuel materials; Inexpensive components; and robotic operation with simple external commands.
The present invention is of a new type of thruster that has demonstrated a 1 nano newton-s MIB in a 100 xcexcs pulse and provides most or all of the desirable features noted above. The thruster can operate continuously, though repetitively-pulsed operation may be convenient for programming force output. Single impulse dynamic range is nearly five orders of magnitude. The invention is of a Laser Plasma Thruster (LPT), preferably employing a semiconductor laser with sufficient brightness to produce a vapor or plasma jet on a surface in vacuum. The invention preferably uses a simple low voltage semiconductor switch to drive the laser. A lens focuses the laser diode output on the ablation target within a very small spot, producing a spark or miniature jet that produces the thrust.
Table 1 illustrates features of the present invention in comparison to prior art thrusters:
There are obvious problems with scaling chemical thruster and other conventional engine types to microscale. Some of these problems are: a) Unpredictable physical regimes are entered when the characteristic dimensions of the nozzles in such devices are so small that gas flow can no longer be characterized by the theory of viscous fluid flow. The assumption of a continuum on which fluid dynamics is based disappears when the mean free path is no longer much smaller than the nozzle diameter. For a 1 xcexcm nozzle, this assumption is strongly compromised at 10 bar pressure. b) Boundary layer effects become dominant in xcexcm-scale nozzles, an important complication for pulsed gas jets and micro-chemical thrusters. As a result, only pulsed plasma thrusters (PPTs) offer realistic alternatives to the laser plasma thruster of the invention for micropropulsion. Micro PPTs must operate from high voltage power supplies, which can increase the mass of a thruster system. They have added difficulties associated with ensuring uniform electrode erosion during operation, unintended electrical breakdown, and ensuring mechanical advance of the plastic dielectric which is ablated by the electrical discharge.
Prior patents illustrative of the prior art include: U.S. Pat. No. 4,036,012, to Monsler et al., entitled xe2x80x9cLaser Powered Rocket Enginexe2x80x9d, discloses a ground-based carbon dioxide gas laser operating at 10.6 micrometers directed at a large mirror on the space vehicle, which is then focused into a gas to produce thrust. U.S. Pat. No. 5,020,411, to Rowan, entitled xe2x80x9cMobile Assault Logistic Kinematic Engagement Devicexe2x80x9d, discloses a plasma thruster in which electricity creates a plasma in a gas to produce thrust. U.S. Pat. No. 5,142,861, to Schlicher et al., entitled xe2x80x9cNonlinear Electromagnetic Propulsion System and Methodxe2x80x9d, discloses a photon rocket providing thrust solely by launching a beam of light. U.S. Pat. No. 5,152,135, to Kare, entitled xe2x80x9cReflector for Efficient Coupling of Laser Beam to Air or Other Fluidsxe2x80x9d, discloses laser propulsion in which laser energy is absorbed xe2x80x9cin air or other fluidxe2x80x9d, with the laser being on the ground or remotely based. U.S. Pat. No. 5,206,455, to Williams et al., entitled xe2x80x9cLaser Initiated Ordnance Systemsxe2x80x9d, discloses a laser-driven detonator. U.S. Pat. No. 5,542,247, to Bushman, entitled xe2x80x9cApparatus Powered Using Laser Supplied Energyxe2x80x9d, discloses a laser driven thruster with laser energy absorbed in a gas (air).
The present invention is of a spacecraft thruster and propulsion method comprising: providing a laser and directing the laser at an ablation target, wherein when the laser is operating, material ablates from the ablation target, thereby generating a thrust vector on the spacecraft. In the preferred embodiment, a semiconductor diode laser is employed or a plurality of semiconductor diode lasers pumping a solid state laser, such as a glass fiber laser. The laser may in fact be located remotely from the spacecraft. The laser is preferably driven with a pulser, most preferably a semiconductor switch. Laser light is focused on the ablation target, preferably within a spot of between approximately 5 and 200 micrometers in diameter for laser light of 1 W power. The ablation target is preferably a substantially light-absorbing (e.g., black) pigment suspended in a polymer, preferably polyvinylchloride and polymethylmethacrylate. For the transmission mode embodiment of the invention, the polymer is supported by a transparent layer (e.g., a second, transparent polymer or an optical glass) through which laser light passes before striking the pigment suspended in the polymer. The ablation target is preferably a tape transported across the focus of the laser. The ablation of the material from the ablation target forms a plasma jet, which need not be confined by a nozzle. The plasma jet is substantially electrically neutral. Continuous thrust of at least approximately 0.4 mN is produced.
A primary object of the present invention is to provide a low mass, efficient, long life, pulsed thruster for low-thrust applications onboard satellites.
A primary advantage of the present invention is that it provides a specific impulse (exhaust velocity divided by the acceleration of gravity) larger than reported by chemical rockets, resistojets or electrically pulsed plasma thrusters because of the high temperature of the vapor or plasma produced by a focused laser.
Another advantage of the present invention is that it provides a wide operating range for impulses generatable by a single device (100,000:1).
An additional advantage of the present invention is that it provides a nearly infinite force range in a single device, because it can be operated in any mode ranging from continuous operation to, for example, one pulse per month.
A further advantage of the present invention is that it can be implemented with a total mass of a few ounces for an engine which can maintain orientation of a nanosatellite or change its orbit.
Still another advantage of the present invention is that it can be extended to larger scale using other kinds of lasers than semiconductor diode lasers on board the spacecraft.
Yet another advantage of the present invention is that it can be extended to larger-scale propulsion systems in which a remote laser of arbitrary power located, for example, on the Earth, the Moon, or another space station propels a distant object or spacecraft using the same physical principles.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.